Method of acoustical study and correction of auditoriums



Aug. 27, 1929. R. F. NORRIS 1,726,499

METHOD OF ACOUSTICAL STUDY AND CORRECTION OF AUDITORIUMS Original Filed April 13, 1927 Patented Aug. 27, 1929.

UNITED STATES 1,726,499 PATENT OFFICE.

RALPH FORBUSH NORRIS, OF MADISON, WISCONSIN, ASSIGNOR TO C. I. BURGESS LABORATORIES, INC., OF MADISON, WISCONSIN, A

CORPORATION OF DELAWARE.

METHOD OF ACOUSTICAL STUDY AND CORRECTION OF AUDITORIUKS.

Original application filed April 18, 1927, Serial No. 188,343. Divided and this application filed January 12,

1929. Serial No. 882,118.

My invention relates generally to the acoustical study and correction of rooms, such as theaters, churches, or other auditoriums, and has particular reference to the control or elimination of echoes in such rooms.

One of the objects of my invention is to provide a method for showing and determining which surfaces of a room, either proposed or existing, cause echoes.

Another object of my invention is to provide a method for showing and determining the direction or directions in which sound is traveling at predetermined given points in rooms, such as theaters, churches, or other auditoriums.

Another object is to provide a method for making a record, as by hotography, of the sound paths andv sound concentrations in rooms.

This application is a division of my copending application Serial Number 183,343, filed April 13, 1927. One form of the invention disclosed herein is claimed in my co-pending application 357,285, filed April 22, 1.929.

Sound waves consist of a traveling succession of relatively high and low pressure areas. These areas move forward in air at approximately 1088 feet per second at ordinary room temperatures. When these waves strike a comparatively smooth and non-porous surface, they are reflected from 'it. As with light waves, if the sound wave strikes a reflecting surface .in a direction normal to its plane, it is reflected back toward its source. If the wave strikes the surface at an angle, it is reflected from the surface at the same angle. After a sound wave has been generated in a room, it is reflected from surface to surface, producing an enormously intricate sound pattern in the room. The laws of sound reflection are like those of light, and every surface that the sound strikes may be considered, in some respects, as a secondary source of sound. As a mirror absorbs some of the light that falls upon it, so surfaces struck by sound waves absorb a portion of the energy in the waves, the percentage being dependent upon the nature of the surface. The result is that the reflected sound in a room becomes absorbed by the successive reflecting surfaces. The length of time that it takes for a sound of standard intensity to be so re duced by absorption that it becomes inaudlble, is called the period of re erberation of the room.

An echo is closely related to reverberation. The human ear is so constituted that it cannot distin uish two sounds less than one-fifteenth o a second apart as two sounds, but receives them as one prolonged sound. If the sounds are more than onefifteenth of a second apart, they are heard by the ear as two distinct sounds.

Sound travels about 1088 .feet per second under ordinary room temperature conditions,

so that if a sound generated at a given point can reach the auditors ear over two aths whose lengths differ by one-fifteenth o 1088 feet, or approximatel 72 feet, the sound will reach the ear of t e auditor as two distinct sounds, 1/15 of a second apart. When this occurs, the sound wave reaching the ear latest is called an echo. When the difference in the length of the two paths is more than 72 feet, the echo will follow the direct sound by a time interval even greater than 1/15 of a second. When one or more echoes are produced in a. room, a confusion of sounds results and the room is described as having poor acoustics. My invention is directed to the method of determining those surfaces which produce confusion echoes.

The usual method now employed for finding echo-producing surfaces in rooms is graphical. In using this method it is necessary to construct scale drawings of crosssections of the room to lay out rays of sound directly the paths is, roughly, 70 feet or more. This method is very laborious and less accurate than my improved method.

The present invention dispenses with the graphical effects and substitutes visual effects which may be produced instantaneously and may be varied at will. The new method providing visual eflects is capable of retaining all the advantages of the graphical method upon photographing the visual effect.

In carrying out my invention, I utilize the similarity as to reflection between sound and light waves. However, the high speed with which light travels (187,000 miles per second) and the ease with which light rays may be photographed. as, for example, by allowing them to act directly on sensitized paper,

or by photographing a surface illuminated by the light rays, make it decidedly advantageous to use light rays in determining the acoustics of a room. This will be explained more in detail hereinafter. It is possible to photograph'the effect of sound waves or pulses but such photographs only show the momentary curvature or contour of the wave front. By taking a succession of photographs, the changing contour of a sound wave may be shown as it strikes obstructions, such as surfaces corresponding to those of a room, or as it interferes with other sound waves, or where it is reflected by surfaces. During the period that the wave front is relatively simple in form, this method may give satisfactory results, but when the sound wave is broken up bystriking various obstructions it becomes diflicult to obtain satisfactory results (see Acoustics in auditorium desi by P. E. Sabine in American Architect, June 18, 1924'). In this method a small modelof a room is erected and successive photographs of a single sound wave emanating from the designated source of sound (such as the stage in a theater) are made. This method is very cumbersome and requires a highly developed technique. It has the further disadvantage that it shows the progress of a single wave only, whereas sound is composed of a rapid succession of waves which will often create acoustic conditions differing widely from the pictures of the single wave.

In my improved method, I replace the sound Waves as emanating from the designated sound source by light rays. I am often able to obtain complete information about the echo-producing surfaces of a room or auditorium in one picture or photograph, and nearly always in three or four. I not only locate the source of the loudest echoes, but I also locate the source of secondar echoes, and I am also able to show visually the direction and intensity of sound at any predetermined point in a room. My method instantaneously shows the sound pattern produced by a multiplicity of sound waves rather than that produced by a single wave front, and therefore shows the normal condition in a room. The simplicity of my method will become more apparent in the following description, taken in connection with the accompanying drawings, in which Figure 1 illustrates a photo-picture made by my process which shows the acoustics of a room, such as a theater auditorium having echoes.

Fig. 2 illustrates a photo-picture of the improved acoustics of the theater of Fig. 1, after being. corrected acoustically.

Fig. 3 shows one form of apparatus used in carrying out my invention.

By way of illustration in one of the commercial methods of carrying out my invention. I first make contours of a room, utilizing both plan views and various horizontal or vertical cross-sections, depending upon the particular problem under consideration. These miniature contours may preferably be from 12 to 24 inches long, but must be faithful reproductions as to the form of the original. When the contour has been reproduced on paper or cardboard, or in other convenient form, a strip of polished metal or other reflecting material, about one to two inches wide, is erected on edge along the plane contour of the room, providing a-fence-like contour that may be considered as a slice-section of the auditorium. The fence may be a single strip, or a number of sections. In Fig. 3, the characteristic portions of the fence are designated by the numerals 1, 2, 3, 4 and 5. It is usually unnecessary to erect a strip for the floor of the room, as I have done at 6, since the audience is on the floor. The audience, together with the seats and carpets, are excellent absorbers of sound and therefore a light-reflecting surface does not represent the true condition. Accordingly, in Fig. 3, the strip 6 is not light-reflecting. It therefore becomes necessary to know the types of surfaces in the room in order to duplicate exactly the conditions in miniature. Surfaces vary in their ability to absorb sound, as is well known to those skilled in the art. Since continuous smooth surfaces are good reflectors of sound and are potential sources of echoes, I simulate these surfaces in the miniatutre contour by leaving the corresponding areas of the reflecting strip polished. The areas which are soundabsorbing are treated so that they do not reflect light, or these sections of the contour strip may be entirely omitted. Occasionally, it only becomes necessary to erect the contour of the ceiling in miniature, since the ceihng is sometimes the only portion having sound-reflecting areas. Side walls may also be sources of echoes. If the room under consideration is the auditorium of a theater, as shown in Fig. 3, there may be a balcony 7,

which must be inserted for reasons which will become apparent. The source of sound is considered to be on the stage 8. A small light 9, preferably an incandescent bulb, is placed at this [)Olllt.

In one application of my invention, after the miniature contour fence is completed, it may be slipped over a sensitized surface, such as a sheet of photographic paper, the strips of polished metal or other reflecting or treated surface forming the contour being at right angles to the sensitized surface. This operation is of necessity carried out in a dark room, or-in properly subdued light. The light 9, if an incandescent bulb, should have a concentrated filament and should be placed so as to illuminate both the contour strip and the entire sensitized surface which corresponds to the area traversed by the sound. This light, being connected to a suitable source of electrical energy, is then flashed. The intensity or length of flash should be such that an concentrations of light which are caused hy the curvature or angle of the metal wall will stand out in contrast to the balance of the sensitized surface. These concentrations of light closely approximate the concentrations of sound in the auditorium. The exposed sensitized surface may then be developed or otherwise treated to fix the image permanently, as in a photo-print.

-In another modification of my process, instead of placing the contour strip on a sensitized surface, I erect it on any uniformly colored plane surface. The light at the point of sound emanation is turned on. The plane surface is illuminated in varying intensity, in roportion to the concentration of sound. his allows the acoustics of the room to be studied, so that the observer may treat the reflecting surfaces. or carry out other ideas and observe immediately the effect of such treatments. Instead of replacing the plane surface with a sensitized surface, as hereinbefore described, the plane surface may be photographed directly by any suitable apparatus in the usual way. Any number of photo-prints may then be made from the negative obtained in this way, the areas of light concentration being light instead of dark, as in the process where the contour is mounted on the sensitized surface.

When it is desired to know in what direction sound is traveling at definite points, as 10 to 29, throughout the room or auditorium, pins, needles, pegs, or similar articles, hereinafter called pins, may be inserted at these points of the surface on which the contour is erected. It is especially desirable to know the source of sound at the points where the audience is located. The pins cast shadows, as shown in Figs. 1 and 2 by dotted lines of which the spacing increases as the shadow fades out. The intense end of a shadow points in the direction of the light source, either direct or reflected. In this way, it is often found that at some points in a room, as in Fig. 1, the audience hears the sound from as many as five points at one time, one sound. slightly after the other. This causes a series of disturbing echoes or reverberations, resulting in poor acoustics. Once the reflecting surface is found, itis a simple matter to apply a highly absorbent acoustical material to it, as is understood by those skilled in the art.

Fig. 1 illustrates the appearance of a photo-picture .made by my method. The picture is cut in the form of the contour. It is the sound pattern obtained for a theater having poor acoustics. In the miniature, or slice model, the light was placed at 9 on the stage 8. The print shows that the ceiling curvature at 1 causes echoes in the audience at 12, 13, 14, 15, 16 and 17. The pins inserted at these points show that the'sound will be heard principally from two points, the stage 8 and the curved surface 1. Since the distance from the stage to the curved surface and back to points 12 to 17 is more than 72 feet greater than the direct distance from the stage, the curved surface. produces bad echoes for the audience, especially in the region from 12 to 17. The curved surface 2 I also causes some echo at 16, 17 and 18, mak

ing the acoustics very poor at 16 and 17. At 18 sound is also heard reflected from section 3' of curved surface 3. Section 3 also reflects the sound into the rear of the balcony, at 22, thereby re-inforcing the sound. This re-inforcement bet-ters the acoustics at 22, since the length of the longer sound path is less than 72 feet more than'the length of the direct path. Fig. 2 shows a photo-picture of the acoustics of the same theater after sound absorbing materials 30, 31 and 32 were installed on the echo producing sections of curved surfaces 1, 2 and 3. The echoes are completely eliminated and the audience on the lower floor hears the sound from only one source, the stage.

As an example in studying the acoustics of an auditorium, it is customary to take one picture of the reflection of the side walls on the floor plan; another picture of the reflection through the longitudinal section of the room; and a third picture on the transverse section of the room. In this way all of the concentrations which cause echoes can be detected, together with the surfaces from which they emanate. In the case ofnew buildings, the angles and curvatures of the surface may be changed to eliminate these echoes, but in the case of buildings already constructed, sound absorbing material may be placed on these surfaces in such a way as to eliminate the echoing effect.

Ill)

. tions of the method and apparatus herein described are, of course, contemplated.- In' the foregoing example, the reflection of light rays proper has been employed to indicate the course of sound by relative intensities of the light. It will be observed that in Fig. 2, the pin 24 casts a shadow 24 which is reflected forwardly as a shadow 24 by the rear wall 5. Thus, a beam of light or a shadow may be emplo ed and its reflection studied. at I claim as .new is:

1. The method of picturing the paths and intensity of sound waves from a given source in a room which comprises makin one or more miniature contour strips 0 said room, treating the surface of said strips to conform to the sound-absorbing characteristics of corresponding sections of said room, placing said contour strips on a plane surface at right angles therewith, placing a light at the source of sound, and photographing said plane surface while illuminated by said light.

2. The method of determining echo-producing surfaces in v a room, which comprises erecting on a plane surface, miniature fence-like sections of contour surfaces of said room corresponding to reflecting surfaces in said room, each surface being 'treated to simulate the sound-reflecting power of said-room surfaces, and placing a light at the source of sound.

3. The method of showing the paths and intensity of sound waves from a given source in a room, which comprises putting a light at the sound source in a miniature cross-section of the contour, of said room mounted on a plane surface; substantially as described.

-. 4. The method of picturing sound-reflecting surfaces in a room, which comprises putting a light at the sound source in a miniature slice-section of said room mounted on a plane surface, and photographing said surface; substantially as described.

5. The combination of steps in the hereindescribed method of determining sound-refleeting surfaces in a room, which comprises placing a source of light rays at the point of sound emanation, in a reduced section of said room, and subjecting sensitized surface to the action of said light rays.

6. The step in the herein-described method of determining the paths and intensity of sound waves at predetermined points in a room, which comprises inserting in a reduced section. of said room obstructions to lightat said points in a plane surface subjected to the action of light rays coming from the point of sound emanation.

" 7. The steps in the herein-described method of determining sound-reflecting surfacesin a room which comprises making a miniature fenced-in contour of a cross-section'of said room and positioning obstructions to light at predetermined points therein.

8. The setps in the herein-described meth- 0d of determining echo-producing surfaces in a room, which comprises erecting on a plane surface, light-reflecting surfaces at right angles therewith, said light-reflecting surfaces simulating the sound-reflecting areas of corresponding sections of a cross section of said room, and providing a con centrated source of light to illuminate said surface.

9. The method of determining the direction and reflection of sound from a given source, which consists in causing light to be emanated at a location corresponding to said sound source, reflecting said light from sur' ing a miniature slice-section of the auditorium with the provision of light-reflecting surfaces as a representation of sound-reflecting surface, using a source of light as a representation of a source of sound, and recordin the reflection of light as an indication 0 the reflection of sound.

"11. The method of determining the possible. acoustic properties of an auditorium having a particular shape, which comprises constructing miniature slice-sections of the auditorium with the provision of light-refiecting surfaces as representative of soundreflecting surfaces, using a source of light as a representation of a source of sound, and recording the reflection of light to particular points in the miniature as representative of the reflection of sound, thereby giving information permitting the proper correction of sound-reflecting surfaces in the particular auditorium.

12. The method of determinin and cor-- recting the acoustic qualities 0 an auditorium of a particular shape, which comprises constructing miniature slice-sections of the auditorium with the provision of light-reflecting surfaces as representative of sound-reflecting surfaces, using a source of light as a representation of a source of sound, recording the reflection of light as an indication of the reflection of sound, selecting the desired reflecting surfaces as representative of sound-reflecting material, and selecting the undesired reflecting surfaces as representative of sound-absorbin material.

13. A method of determining the location or character of sound receiving surfaces in a proposed auditorium, which comprises arranging light reflecting surfaces in miniature 'as representative of surfaces defining the auditorium, placing a source of light as a representation of a source of sound in a particular relation to observation points within the auditorium, the sound-reflecting or absorbing character of such surfaces being determined by the direction and the intensity of the light from the source to said particular points.

14. The method of determining sound reflection in a room, which comprises making a miniature model of a s1ice-section of the room with light-reflecting surfaces, providing a light as a representation of a source of sound and projecting a beam from said light onto said surfaces for reflection thereby.

In testimony whereof, I have subscribed my name.

RALPH FORBUSH NORRIS; 

